Effect of Size-Dependent Thermal Instability on Synthesis of Zn2SiO4-SiOx Core–Shell Nanotube Array

NANO EXPRESS

Effect of Size-Dependent Thermal Instability on Synthesis of Zn 2SiO 4-SiO x Core–Shell Nanotube Arrays and Their Cathodoluminescence Properties

Chun Li ?Yoshio Bando ?Benjamin Dierre ?Takashi Sekiguchi ?Yang Huang ?Jing Lin ?Dmitri Golberg

Received:21December 2009/Accepted:28January 2010/Published online:10February 2010óThe Author(s)2010.This article is published with open access at https://www.sodocs.net/doc/082072471.html,

Abstract Vertically aligned Zn 2SiO 4-SiO x (x \2)core–shell nanotube arrays consisting of Zn 2SiO 4-nanoparticle chains encapsulated into SiO x nanotubes and SiO x -coated Zn 2SiO 4coaxial nanotubes were synthesized via one-step thermal annealing process using ZnO nanowire (ZNW)arrays as templates.The appearance of different nanotube morphologies was due to size-dependent thermal instability and speci?c melting of ZNWs.With an increase in ZNW diameter,the formation mechanism changed from decom-position of ‘‘etching’’to Rayleigh instability and then to Kirkendall effect,consequently resulting in polycrys-talline Zn 2SiO 4-SiO x coaxial nanotubes,single-crystalline Zn 2SiO 4-nanoparticle-chain-embedded SiO x nanotubes,and single-crystalline Zn 2SiO 4-SiO x coaxial nanotubes.The difference in spatially resolved optical properties related to a particular morphology was ef?ciently documented by means of cathodoluminescence (CL)spectroscopy using a middle-ultraviolet emission at 310nm from the Zn 2SiO 4phase.Keywords Nano-template áCore–shell nanotube á

Cathodoluminescence áZinc Silicate áRayleigh instability áKirkendall effect

Introduction

Nanotubes made of carbon and diverse inorganic com-pounds have continuously attracted signi?cant attention due to their unique fundamental physical properties and many potential applications [1–3].Inorganic nanotube generation strategy can generally be classi?ed into two categories:?rst,one-step self-organization such as self-rolling and/or Ostwald ripening;second,two-step tem-plate-based fabrication through either scari?cation or recently developed solid-state reaction utilizing the Kir-kendall effect [4–6].From a viewpoint of ?nal device integration,aligned nanotube arrays are highly desirable.In addition to the direct epitaxial growth of nanotube arrays on lattice-matched substrates,a solid-state reaction under thermal annealing and using readily available nanostruc-ture arrays as templates could be an ef?cient way to gen-erate novel chemically complex,multiphase nanotube arrays.However,due to a considerable increase in the surface-to-volume ratio with decreasing a nanomaterial size,size-dependent thermal stability and melting of the nanostructured templates during annealing must be care-fully addressed.

ZnO nanowire (ZNW)arrays are one of the most com-mon aligned nanostructures owing to their naturally pref-erable epitaxial growth at a relatively low temperature.They have widely been utilized as templates for the syn-thesis of lattice-matched GaN and SiC nanotubes,[7,8]ZnO-related semiconducting heterojunctions [9,10],and ZnO-based ternary compound nanostructures [11].In most cases the latter have been considered to be even more important than pure-phase ZnO ones [4–6].Although solid-state reactions by thermal annealing based on a ZNW template have been used to synthesize ZnO-based ternary compound nanotubes,[4–6,11]no research has been

Electronic supplementary material The online version of this article (doi:10.1007/s11671-010-9556-7)contains supplementary material,which is available to authorized users.

C.Li (&)áY.Bando áY.Huang áJ.Lin á

D.Golberg World Premier International Center for Materials

Nanoarchitectonics (MANA),National Institute for Materials Science,Namiki 1-1,Tsukuba,Ibaraki 305-004,Japan e-mail:LI.Chun@nims.go.jp;whulic@https://www.sodocs.net/doc/082072471.html, B.Dierre áT.Sekiguchi

Advanced Electronic Materials Center,National Institute for Materials Science (NIMS),Namiki 1-1,Tsukuba,Ibaraki 305-0044,Japan

Nanoscale Res Lett (2010)5:773–780DOI 10.1007/s11671-010-9556-7

carried out on the in?uence of size-dependent template thermal stability on the?nal characteristics of such tubes.

In this study,this phenomenon was thoroughly demon-strated for the case of Zn2SiO4.While employing a one-step solid-state reaction and the ZNW array templates, vertically aligned Zn2SiO4-SiO x core–shell nanotube arrays (ZSO),i.e.,Zn2SiO4-nanoparticle chains encapsulated in SiO x nanotubes,SiO x-coated polycrystalline and single-crystalline Zn2SiO4coaxial nanotubes were simultaneously obtained.Furthermore,a cathodoluminescence(CL)study, as a noninvasive and high spatial-resolution characteriza-tion tool,was employed to detect and analyze local struc-tural and optical properties of the nanostructures.The structural and optical differences were effectively identi-?ed by transmission electron microscopy(TEM)paired with CL spectroscopy.Finally,the size-dependent thermal instability induced formation mechanism was proposed. Experimental Methods

Synthesis Methods

The ZNW templates were grown on a ZnO-?lm-coated silicon substrate using a vapor phase transport,as we pre-viously reported[12].The synthesis of ZSO nanotube arrays was carried out in a vacuum tube furnace with an outer diameter of24mm and a length of1,200mm.A0.2g powder mixture of SiO2(Alfa Aldrich,99.9%),activated carbon(as a reductant),and Si with equal molar ratios was placed at the center of the tube.A piece of the ZNW tem-plate was placed downstream of the tube at*14cm away from the source.The tube was sealed and evacuated to a base pressure of*2Pa.The furnace was then heated to 1,100°C at a rate of24°C min-1and kept at this temperature for2h.The local temperature of the substrate was about 1,000°C due to the temperature gradient along the tube furnace.A constant?ow of high-purity Ar gas was fed into the tube at a?ow rate of80sccm(standard cubic centi-meters per minute)and a pressure of100Torr throughout entire heating/cooling.After the furnace was naturally cooled to room temperature,the surface color of ZNW templates changed from black to white–gray. Characterization Tools

The ZNW templates and ZSO samples were characterized by a powder X-ray diffraction(XRD;Rigaku,Ultima III, 40kV/40mA with Cu K a radiation),a scanning electron microscope(SEM;JEOL,JSM-6700F),a high-resolution ?eld-emission transmission electron microscope(TEM; JEOL,JEM-3000F),a high-angle annular dark-?eld scan-ning transmission electron microscope detector(HAADF-STEM)(JEOL JEM-3100FEF),and an energy-dispersive X-ray spectrometer(EDX).CL spectra were recorded at room temperature in an ultrahigh-vacuum SEM with a Gemini electron gun(Omicron,Germany)at an accelerating voltage of10kV and a current of1nA.

Results and Discussion

Vertically aligned ZNW arrays with an underlayer of interconnected nanowalls were grown on a ZnO buffer layer[12].The ZNWs have typical diameters of*30–150nm and lengths of*10l m.Cross-sectional view SEM images(Fig.1a,c)revealed that the ZSO arrays keep the vertical alignment peculiar to the ZNW templates and protrude from the underlayer nanowall networks.The top-view SEM images(Fig.1b,d)clearly indicate that the nanotube diameters are larger than those of ZNW due to SiO x coatings.The originally separated ZNWs stick to each other after thermal annealing.Hollow tube morphologies can be clearly seen from the top-view SEM image of the ZSO sample after scratching off the surface nanotubes (inset of Fig.1d).The XRD spectrum of the ZNW arrays displays only one strong(002)peak due to their high c-axis orientation growth.A strong diffraction peak at34.02o and other four relatively weak peaks in the XRD spectrum (Fig.2)of the ZSO sample can be indexed to a rhombo-hedral Zn2SiO4crystal structure with the lattice constants of a=b=1.394nm and c=0.9309nm(JCPDS Card: 08-0492).The ZnO(002)peak coming from the ZnO buffer layer can also be seen.

Detailed crystal structure and compositional analyses of the ZSO samples were carried out by TEM and EDS.Three types of tube morphologies,i.e.,discrete particles encap-sulated tubes,hollow tubes,and tubes only partially?lled with nanoparticles,were observed(Fig.3a).All the nano-tubes including their tip-ends have a uniform SiO x(x\2, as measured by EDS,see supporting Figure S1)shell layer (the inset TEM image of Fig.3b).High-magni?cation and high-resolution TEM observations(HRTEM)and selected area electron diffraction(SAED)patterns reveal that the as-synthesized ZSO samples consist of three types of tubular morphologies depending on their diameters(Fig.3b),as shown in Fig.3c,f,and i,respectively.The nanotubes (*20%)with diameters*50–100nm have usually an amorphous SiO x(*20nm)outer shell and an inner tube (diameter*30nm)made of polycrystalline Zn2SiO4par-ticles(Fig.3c).The polycrystalline nature of the inner tube was con?rmed by HRTEM imaging and SAED patterns (Fig.3d,e).The weak lattice fringes in Fig.3e with a d-spacing of 1.62,2.53, 2.63,and 2.83A?can also be observed.These correspond to a rhombohedral Zn2SiO4 phase(r-Zn2SiO4)and(315),(042),(410),and(113)planes

of it.About 40%of nanotubes with diameters of *90–160nm have a unique morphology of single-crystalline Zn 2SiO 4-nanoparticle chains encapsulated into a SiO x nanotube.The SAED pattern in Fig.3h recorded from an embedded rod-shaped nanoparticle can be indexed to the [2

ˉ21]zone of r -Zn 2SiO 4and con?rms the single-crystalline character of the particle.The nanoparticles are partially

surrounded by a thin amorphous SiO x layer (*5nm thick),as seen in Fig.3g.The other *40%nanotubes with outer diameters larger than *150nm were found to be SiO x -coated single-crystalline Zn 2SiO 4nanotubes.The corre-sponding HRTEM image and SAED pattern (Fig.3j,k)verify that an r -Zn 2SiO 4nanotube is a single crystal.The

adjacent lattice fringes separated by 2.83,4.02,and 7.02A

?and labeled in Fig.3d,g,and j correspond to the inter-planar distances of (113),(300),and (110)planes in Zn 2SiO 4,respectively.This fact is consistent with the XRD results shown in Fig.2.

The spatial distribution of Zn,Si,and O species was mapped using corresponding EDX K a emissions in the HAADF-STEM mode.Due to a large difference in atomic numbers between Zn and Si,a HAADF-STEM image in Fig.4a clearly reveals the typical structural features of a Zn 2SiO 4-nanoparticle-encapsulated SiO x nanotube,and a SiO x -coated Zn 2SiO 4coaxial nanotube.As shown in the spatially resolved elemental maps in Fig.4,Zn species are only present within encapsulated nanoparticles and the inner shell of the Zn 2SiO 4-SiO x coaxial nanotube.

CL spectroscopy combined with SEM imaging has been proven to be a powerful tool for probing the local char-acteristics of low-dimensional materials due to its

high

Fig.1SEM images of a ,b a ZNW template and c ,d a ZSO sample.The inset top-view SEM image shows the ZSO sample after scratching surface nanotubes out of the substrate.The scale bars in a –d are 1l m and for the inset is 500nm.a ,c cross-sectional view images,b ,d top-view

images

spatial resolution (a submicrometer range)[13,14].This makes CL a valuable nondestructive tool for studies of inhomogeneities in nanostructures caused by doping or growth condition variations.We attempted to use CL spectroscopy to detect and analyze the Zn 2SiO 4phase in the ZSO samples.The CL spectrum recorded from the ZSO sample cross-section is shown in the inset of Fig.5b.The emission peak at 310nm (4.0eV)with a shoulder at 280nm (4.4eV,see ?tting curve of supporting Figure S2),which is well below the band gap 5.4eV of Zn 2SiO 4,is attributed to the radiation recombination from the Zn 2SiO 4phase,considering that the similar emission peak at 300nm (4.13eV)was previously reported for the Zn-Zn 2SiO 4nanocables [15].The 380nm emission peak can be ascribed to the near-band-edge recombination of a crystalline ZnO buffer layer.These two sharp emission peaks may result from the single-crystalline characters of Zn 2SiO 4and ZnO phases,respectively.From the CL spectra recorded in different spots,indicated in Fig.5b,the intensity of peaks at 380nm shows a dramatic decrease from the ZnO buffer layer toward the nanotubes;the opposite trend is in effect for the emission at 310nm.This indicates that all crystalline ZNWs have been totally con-sumed during annealing.The 380nm emission only comes from the ZnO buffer layer.The weak violet emission at 440nm can be rationally assigned to a SiO x phase [16],and the broad emission band at 550nm can be ascribed to the Zn 2SiO 4phase.Note that there is also report on the assignment of 440and 550nm emission bands to the related defect emission of ZnO [17].The corresponding CL emission images recorded for each peak are shown in Fig.5c–f.The 310nm CL emission image (Fig.5c),combined with the 550nm emission image in Fig.5f,shows the inhomogeneous distribution of the Zn 2SiO 4phase,i.e.,stronger intensities for the underlayer nanowalls (which have a larger size compared to the top

nanotube

Fig.3a Low-magni?cation TEM image of nanotubes;b Histogram of the diameter distribution based on 70

randomly selected nanotubes.The inset shows tip-end TEM images for the three types of nanotubes.c High-magni?cation TEM images,type I:a SiO x -coated polycrystalline Zn 2SiO 4nanotube,f type II:a SiO x nanotube periodically encapsulated with single-crystalline Zn 2SiO 4

nanoparticles,and i type III:a SiO x -coated single-crystalline Zn 2SiO 4nanotube;d ,g ,and j corresponding HRTEM images taken from the framed areas marked in (c ),(f ),and (i ),respectively;(e )and (k )

corresponding SAED patterns taken from the nanotubes shown in (c )and (i ),respectively.(h )SAED pattern taken from the encapsulated nanoparticle in (f ).The diameter distributions for type I,II,III nanotubes are labeled in different sectors in (b )

portions).The uniform 440nm emission image is consis-tent with the TEM observation of a homogeneous SiO x coating.The assignment of 310and 550nm emissions was further con?rmed by the emission from the interface between a ZnO buffer layer and a Si substrate,as indicated by arrows in Fig.5c,f.One can expect that the reaction between the ZnO ?lm and the surface SiO x layer on a Si substrate can lead to Zn 2SiO 4phase formation during high-temperature annealing [18].

The further high-magni?cation CL images were recor-ded from representative nanotubes dispersed on a TEM grid.Owing to the high sensitivity and high spatial reso-lution of the CL spectroscopy mapping,the nanotubes with almost same elemental compositions but different mor-phologies can readily be indenti?ed at a large scale.As shown in Fig.6c,strong 310nm CL emission comes from both the embedded Zn 2SiO 4nanopaticles (type II)and Zn 2SiO 4nanotubes of larger size (type III),while smaller SiO x -coated Zn 2SiO 4nanotubes (type I)only show hardly detectable emission.These results are in a good agreement with TEM characterization.Also,they further con?rm the above-mentioned peak assignment.

It will be of interest to investigate the formation mechanism during simultaneous generation of different tube structures,especially regarding the Zn 2SiO 4-nano-particle chains,for the ?rst time observed here in a ternary compound nanomaterial.We believe that nanoscale ther-modynamics of ZNW is an important factor.It has been experimentally demonstrated that the melting point of Cu,Zn,and Sn nanowires will signi?cantly decrease with a decrease in wire diameter [19–21].Based on molecular dynamics simulations,the same diameter dependency has also been found for GaN nanowires [22].In addition,under heating in air,ZnO nanorods start to melt at 750°C,[23]much lower temperature than the melting point of a bulk form.Therefore,it is reasonable to assume that the melting point of ZNW would also decrease with diameter decreasing.

Based on the above-mentioned assumption and taking the results of XRD,TEM,EDS,and CL measurements,we conclude that the factors responsible for the different tube morphologies are the size-dependent melting behavior of ZnO and the competition between surface and volume diffusions.Since the Si atom has nearly the same radius as the Zn atom and the bonding energy of the Si–O bond (185kJ/mol)is about two times higher than that of the Zn–O bond (92kJ/mol),it is easy for ZnO to diffuse into SiO and to form a Zn 2SiO 4phase,as reported by Wang and Zhou [11,25].It is also noted that the ZNW template was totally decomposed in a control experiment under the same heating process but without introducing a source powder.The formation mechanism here is illustrated in Fig.7,and the three predominated effects decomposition of ‘‘etch-ing’’,the Rayleigh instability,and the Kirkendall solid-state reaction are highlighted for the nanotubes of type I,II,and III,respectively.During the temperature increase,the Si–O vapor (sublimated from the source)can quickly be transported and deposited on the nanowire surfaces,pre-venting their decomposition.Rapid heating quickly drives the system to high temperature.The ZNWs with

smaller

Fig.4a HAADF-STEM image of two typical Zn 2SiO 4

nanotube morphologies,a SiO x -shelled Zn 2SiO 4nanochain-encapsulated nanotube (left )and a SiO x -coated Zn 2SiO 4

nanotube (right );b ,c ,and d the corresponding spatially resolved elemental EDS maps of Zn,Si,and O

diameters (\60nm,estimated from the above TEM sta-tistical analysis),which possess lower melting tempera-tures,prefer to decompose into Zn vapor and O 2rather than to react with Si forming Zn 2SiO 4.The ZnO de?ciency and SiO x richness at the interface lead to the formation of a thin polycrystalline Zn 2SiO 4layer on the inner surface of

the

Fig.5a Cross-sectional view SEM image of a ZSO sample and corresponding room-temperature CL spectrum in (b );c ,d ,e ,and f CL images of the array under the 310,380,440,and 550nm emissions,

respectively

Fig.6a ,b ,and c TEM and SEM images,and corresponding 310nm CL emission image of representative nanotubes dispersed on a TEM grid.Three nanotube types are labeled as I,II,and III,and indicated

by arrows ,respectively.Note that type I nanotubes consist of two nanotubes of small diameter.The scale bars are 200nm

SiO x shell.The ZNWs with medium diameters (*60–120nm)prefer melting rather than decomposition.In accord with the Rayleigh instability effect,[19,24]the ZnO liquid cylinder of radius r is unstable to radius per-turbations whose wavelength l exceeds the circumference of the cylinder.The cylinder thus decreases its total energy and breaks into nanoparticles under surface tension.As the ZnO liquid starts to shrink,the Si/O surface diffusion will rapidly increase around the curved portions,since the surface diffusion of atoms is proportional to the gradient of the wire surface curvature at a certain temperature [26].Subsequent volume diffusion at the interface and sub-sequent reactions lead to single-crystalline Zn 2SiO 4-nano-particle chains encapsulated into SiO x nanotubes.The ZNWs with larger diameters ([120nm)will be in a ther-mally unstable solid state.Unequal diffusion rates for the outward Zn/O and inward Si/O,known as the Kirkendall diffusion,[4–6]produce a hollow interior along the struc-ture length.The SiO x coating is thicker than necessary for complete consuming of the interior ZnO to form a new phase.As a result,SiO x -coated single-crystalline Zn 2SiO 4coaxial nanotubes are formed.Note that due to the diam-eter nonuniformity of an individual wire,nanotubes with partial nanoparticle encapsulation may also form,as shown in Fig.3a.

It is worth mentioning that by using plasma-enhanced chemical vapor deposition (PECVD)of a thin amorphous Si ?lm (*10nm)on the top half of a ZNW template followed by vacuum annealing,Zhou et al .[11]fabricated vertically aligned Zn 2SiO 4nanotube/ZnO nanowire het-erojunction arrays.Our results undoubtedly demonstrate that prior to ZnO template utilization for reliable making

ternary compound nanotubes,problems associated with their thermal instability must be seriously taken into account and solved.The good news is that due to many diameter-controllable synthesis methods of ZNWs reported to date,?nding templates with uniform diameters for the subsequent unique one speci?c ternary nanotube syntheses looks highly plausible.Furthermore,the Rayleigh insta-bility applied to ZNW provides a structuring technique that produces long chains of ZnO-based compound semicon-ducting nanospheres which may ?nd potential applications in nanoscale photonic devices [27].

In conclusions,vertically aligned Zn 2SiO 4-nanoparticle chains encapsulated into SiO x nanotubes,SiO x -coated polycrystalline and single-crystalline Zn 2SiO 4coaxial nanotubes were simultaneously fabricated by a one-step solid-state reaction using ZNW array templates.It is found that the nanotubes of different morphologies can be easily identi?ed by CL spectroscopy mapping technique.The apparent size-dependent nonuniform nanotube generation was due to the strong size-dependent thermodynamic behavior of ZNWs.With diameter increasing of starting ZNW,the formation mechanism changes from decompo-sition of ‘‘etching’’,to Rayleigh instability,and then to Kirkendall effect,resulting in polycrystalline Zn 2SiO 4-SiO x coaxial nanotubes,single-crystalline Zn 2SiO 4-nanoparticle-chain-embedded SiO x nanotubes,and single-crystalline Zn 2SiO 4-SiO x coaxial nanotubes,respectively.

Acknowledgments This work was supported by the World Premier International Center for Materials Nanoarchitectonics (MANA)Pro-ject tenable at the National Institute for Materials Science (NIMS),Tsukuba,Japan.The authors thank Drs.A.Nukui,M.Mitome,and Mr.K.Kurashima for the continuous technical support and kind

help.

Fig.7Schematics of the

formation mechanism for three discovered types of tubular structures

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